Aldéhydes

Aldéhydes are a class of organic compounds that contain the aldehyde functional group, characterized by the carbonyl group bonded to a hydrogen atom at the end of a carbon chain (-CHO). The general formula is R-CHO, where R can be a hydrogen or a hydrocarbon substituent. The carbonyl carbon is typically highly reactive, making aldehydes versatile intermediates in organic synthesis.

Common aldehydes include formaldehyde (methanal, H2C=O), acetaldehyde (ethanal, CH3CHO), and propanal (CH3CH2CHO), as well as benzaldehyde (C6H5CHO). Nomenclature uses the suffix -al, with the parent chain including the aldehyde carbon; the simplest aldehyde has its name derived from the corresponding alkane.

Aldehydes are polar due to the carbonyl group, giving them higher boiling points than similar alkanes but generally lower than alcohols. Small aldehydes are miscible with water, while solubility decreases as the hydrocarbon portion grows. They are typically reactive toward nucleophiles at the carbonyl carbon and can undergo oxidation to carboxylic acids or reduction to primary alcohols. Aldol reactions enable carbon–carbon bond formation, and aldehydes lacking α-hydrogens can undergo base-catalyzed Cannizzaro reactions. Informational tests such as Tollen’s or Fehling’s/Benedict’s can distinguish aldehydes (which reduce the reagents) from ketones.

Production and uses: aldehydes are commonly formed by oxidation of primary alcohols or by hydroformylation of alkenes. Formaldehyde is industrially important for resins, coatings, and disinfectants; acetaldehyde is a precursor in perfumes, solvents, and chemical synthesis; benzaldehyde is valued for flavor and fragrance applications.

Safety and environmental notes: aldehydes can be irritants and some, notably formaldehyde, are hazardous with potential carcinogenic effects; handling requires appropriate controls and ventilation.